Various types of vehicles may include a towing or tow haul mode that allows the vehicle to maintain speed while going downhill by using engine friction or engine braking to reduce the need for the driver to use the service brakes. This mode may be engaged by the driver when the vehicle is towing cargo in hilly terrain. The tow haul mode may attempt to maintain vehicle speed by progressively shifting to a lower gear (higher gear ratio) as the vehicle accelerates downhill. This mode usually does not request increased acceleration from vehicle controls if the vehicle is slowing.
For vehicles having a transmission with a torque converter, the torque converter bypass clutch is usually locked when downshifting to a lower gear to produce more negative torque at the wheels if the vehicle is accelerating downhill. Downshifting results in increased engine speed thereby producing higher friction torque as well as a higher ratio multiplication of engine torque to the vehicle wheels thereby providing more negative torque at the wheels. When the vehicle is slowing to a stop, the torque converter bypass clutch is unlocked during downshifts. Opening of the torque converter bypass clutch effectively separates the engine speed from vehicle speed and the engine transitions to speed control while the vehicle slows down smoothly without driver input.
In hybrid vehicles having a transmission with a torque converter and operating in a towing mode, the torque converter bypass clutch may be locked during downshifts even when the vehicle is slowing down to a stop to facilitate regenerative braking to increase energy efficiency. However, the resulting deceleration when the transmission downshifts as the vehicle slows down to a stop may be considered objectionable drivability for some drivers or occupants.